A salt dome is a mound or column of salt that has risen toward the surface because it has a density that is lower than the rock above it. When a layer of salt is deposited on the floor of an evaporating body of water, it has a specific gravity of about 2.2. Other sedimentary rocks such as shale and limestone have lower specific gravities when they are deposited because the mud that they form from contains a significant amount of water. As the depth of burial increases, the specific gravity of salt remains about the same, but the specific gravity of shale and limestone increases as the water is squeezed from their pore spaces. Eventually the shale and limestone might have a specific gravity of 2.4 to 2.7, which is significantly higher than the salt. That creates an unstable situation where a lower specific gravity material such as salt is capable of behaving like a fluid and can move upwards.
As the salt moves up towards the surface, it can penetrate and/or bend strata of existing rock with it. As these strata are penetrated, they are generally bent slightly upwards at the point of contact with the dome, and can form pockets where petroleum and natural gas can collect between impermeable strata of rock and the salt.
Salt domes were almost unknown until an exploratory oil well was drilled on Spindletop Hill near Beaumont, Tex. in 1900 and completed in 1901. Spindletop was a low hill with a relief of about 15 feet where a visitor could find sulfur springs and natural gas seeps. At a depth of about 1000 feet, the well penetrated a pressurized oil reservoir that blew the drilling tools out of the well and showered the surrounding land with crude oil until the well could be brought under control. The initial production from the well was over 100,000 barrels of crude oil per day—a greater yield than any previous well had ever produced. Thus, from the earliest days of exploration, prospectors have associated salt with oil and gas wells.
In fact, salt is one of the most effective agents in nature for trapping oil and gas. It is a ductile material allowing it to move and deform surrounding sediment to create traps; yet, salt is also impermeable to hydrocarbons and acts as a seal. Salt's plasticity allows it to move in an upward motion creating pockets where crude oil and natural gas can seep in and remain trapped once the salt eventually dries. As such, most of the hydrocarbon in North America is trapped in salt-related structures. However, until the 1980s, it was uncommon for explorers to seek out hydrocarbons under the salt structures. An increased focus on imaging salt structures has opened the door for exploration below these structures.
Seismic prospecting techniques are commonly used to search for and evaluate of hydrocarbon deposits located in subterranean formations, including salt structures. In seismic prospecting, seismic energy sources are used to generate a seismic signal, which propagates into the earth and is at least partially reflected by subsurface seismic reflectors. Such seismic reflectors typically are interfaces between subterranean formations having different elastic properties. The reflections are caused by differences in elastic properties, specifically wave velocity and rock density, which lead to differences in impedance at the interfaces. The reflections are recorded by seismic detectors at or near the surface of the earth, in an overlying body of water, or at known depths in boreholes. The resulting seismic data is processed to yield information relating to the geologic structure and properties of the subterranean formations and potential hydrocarbon content.
However, many difficulties exist in imaging salt structures, particularly in both its plastic properties and in the three dimensional structure of the salt dome. For instance, the top part of the salt structure almost always overshadows the traps, essentially putting them in a “shadow zone”, most particularly beneath salt flanks. Additionally, the salt acts as a barrier and scatters the seismic waves used to build an image of the subsurface. Thus, accurate visualization of the salt structure requires time-consuming computer interpolation and rendering.
To maximize the understanding of the position of seismic reflectors near and below salt structures, it is necessary to build an accurate model of the salt structure itself, especially the bottom of the salt. Today, the accuracy of a salt model is examined according to the quality of the migrated seismic imagery under the salt structures. If an area of seismic has poor quality or indicates questionable seismic features, the salt model is modified slightly, and seismic data re-migrated using that new salt model. This iterative process is repeated in an attempt to maximize the quality (and from that the accuracy) of the migrated seismic record. It is critical to have confidence in the geology surrounding salt structures for effective prospect evaluation.
The modify-remigrate-analyze step sequence is often referred to as an iteration. This iterative process is described below:
1. Make an estimate of the 3D salt model.
2. From the estimated 3D salt model, apply a migration technique to generate a new migrated seismic data.
3. Perform a qualitative analysis on the migrated seismic data, identifying areas that suggest the salt model above is incorrect or determining that no further changes are needed.
4. Make small changes to the 3D salt model in an effort order to improve the quality of the migrated seismic data.
5. Go to step 2 and repeat.
The iterative process is quite slow and current methods of imaging salt do not permit fast and precise changes in the salt model in Step 4. Model changes are currently made by careful line-by-line reinterpretation of all affected horizons using standard 2D interpretation tools. This is an extremely time-consuming process, and produces mixed results.
Some commercially available software such as LANDMARK® (from HALLIBURTON®) and PETRAL® (SCHLUMBERGER®) requires conversion of top/bottom horizon pairs to a triangle mesh representing the boundary. The geometry model is then simplified and subsequently interactively deformed by manipulating the points on the existing geometry. The result is then converted back to horizon pairs. This approach causes loss of precision over the whole structure in order to make a small change.
GX Technologies™ has developed a technique in which a geometric surface is interactively pushed and pulled into a desired shape. Other tools perform basic deformation of single horizons, but lack the ability to deform the model horizontally, work well at areas of high dip, and/or lack the information about 3D salt boundary.
Thus, there exists a need for methods of accelerating the salt modeling process, while maintaining an accurate depiction of the salt structure. Ideally, the method will facilitate making small changes in the 3D salt model to improve the model.